The background section is intended to provide a background or context to the various embodiments described herein, and within the claims. The background may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application, and is not admitted to be prior art by inclusion in this section.
Primary and rechargeable lithium secondary batteries have been the object of considerable research and development. The aim was to develop a low cost battery, with high energy density and good electrochemical performance. With this in mind, a large number of battery designs have been developed to comply with different applications such as portable products, electric vehicles (EV) and start-light-ignition (SLI) vehicles. The focus to date has been on Li-ion batteries that use a lithium metal oxide, such as LiCoO2, Li(Ni1/3Co1/3Mn1/3)O2, Li(Ni0.8Co0.15Al0.05)O2, and LiNi0.5Mn1.5O4, or phosphate, such as LiFePO4 and LiMnPO4, as cathode materials, and metal oxides, such as Li4Ti5O12, TiO2, SiOx, and SnO2, as anode materials.
In some cases, electroactive materials used as the cathode or anode in lithium batteries have poor electronic conductivity, and as a consequence, their electrochemical performance and rate capability suffers. Examples of such materials include Li4Ti5O12, SiOx, A2MTi4O16 (A: Li, Na, K) (M: Ba, Ca, Sr, Mg, Zn), high energy composite electrodes Li2MnO3—LiMO2 (M: Mn, Co, Ni), LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4. These materials exhibit poor rate capability and poor utilization at high electrode loading densities and therefore are typically ill-suited for applications that require high energy and high power such as plug-in hybrid electric vehicles (PHEV) and EVs.
To overcome this limitation, lithium metal oxides with layered or spinel structures, such as Li1.2Mn0.5Ni0.176Co0.1O2, and metal oxides, such as TiO2 and SnO2, have been proposed as cathode and anode materials, respectively. However, despite their large capacity, such oxides exhibit lower electrical conductivity that has restricted their use to low-power applications.
Due to the conductivity limitations of some electroactive materials, a cathode of choice is LiCoO2, and an anode of choice is either carbon or Li4Ti5O12. However, LiCoO2 operates at only 4.2V (versus Li), with a capacity of 150 mAh/g, and the high price and toxicity of cobalt is prohibitive for large size batteries. With respect to the anode, Li4Ti5O12 has a capacity of 160 mAh/g, which is insufficient to meet PHEV and EV capacity requirements.
Carbon is known as an electrical conductor and is used to increase the electronic conductivity of materials like LiFePO4. In general, carbon coating of oxide materials is carried out using a pyrolysis process that forms a thin layer of pyrolitic graphite on the surface of particles and allows for an even distribution of electrons on the surface of each particle. Carbon coating of electroactive materials to counter low electrical conductivity, for example in the case of LiFePO4, has resulted in significant improvement in the rate capability. With 5 wt % carbon coating, the electrical conductivity of LiFePO4 increases from 10−7 to 10−3 mS. In this case, the addition of carbon is relatively straightforward due to the stability of Fe2− in a reducing atmosphere (e.g., H2, CO, CO2 or even Ar, N2 or He with carbon). However, the use of carbon, or a reducing agent, for coating other oxide materials has been very limited due to the poor stability of metal ions under reducing conditions. In most cases, an attempt to coat a layered metal oxide with carbon results in partial, or total, reduction of the oxide.